Blending devices for blending various types of solid particulate ingredients, such as plastic pellets, are well-known in the art. Generally, these devices include a number of ingredient hoppers, each of which discharge ingredients into individual metering units. These metering units typically include a metering auger, the rotational speed of which can be varied to control the flow rate of the individual ingredients. The metering units discharge individual ingredients into some sort of common hopper at independently controllable feed rates which can be varied to produce the desired blend of individual ingredients. Typical feed devices are shown, for example, by U.S. Pat. No. 1,757,341 to Smit, U.S. Pat. No. 2,957,608 to Wahl, and U.S. Pat. No. 3,684,082 to Wardell.
Advances in material and compounding technologies have led to the production of ingredients for blending with a higher concentration of additive materials, such as pigments and antioxidants. These highly concentrated ingredients are usually more costly than the other materials in the blend and typically constitute a very low percentage of the total blend, which requires operation of at least one of the metering units at very low flow rates, often at 0.2 lbs./hr or less. When operating a metering unit at such low rates, it is desirable to achieve as small a short-term flow rate variation as possible. Ideally, it would be preferable to eliminate such short-term flow rate variations. In many situations it is necessary or important to maintain at least a minimum flow rate, despite the undesirable short-term fluctuations in flow rate. In such situations, it has been common to set the nominal or target rate well above the required minimum flow rate in order to ensure that the minimum rate is maintained despite short-term fluctuations in flow rate. Substantial cost savings could be achieved if the nominal or target rate could be set closer to the minimum rate while still ensuring that the minimum flow rate is maintained.
It has been discovered by Applicant that one of the primary causes of flow rate fluctuations at low flow rates is the rotation of the metering auger itself. Existing prior art metering devices typically are fed by circular, square or rectangular ingredient hoppers with the lower portion having converging walls mounted directly to the metering auger housing, as demonstrated by U.S. Pat. No. 1,757,341 to Smit. In this arrangement, the weight of ingredient particles in the hopper is supported almost entirely by the sloped walls of the ingredient hopper, with only a small fraction of the weight of the ingredients being reactive with (i.e. affected by) the auger. It has been found that upward forces resulting from the rotation of the auger cause perturbations in this small reactive volume of particulate matter just above the auger. At low metering rates these perturbations are sufficient to cause fluctuations in material feed to the auger, thus resulting in uneven operation.
Accordingly, it can be seen that a need yet remains for a blender apparatus capable of precise low-rate metering, which minimizes perturbations in the low-rate metering. It is to the provision of such a blending apparatus capable of providing such precision low-rate metering that the present invention is primarily directed.